The invention relates to a method for producing a monolithic, porous, ceramic shaped body which is predominantly used as a supporting material for porous, inorganic and/or organic membranes for the flow filtration (cross flow) of liquids and gases.
According to the prior art, porous oxide ceramic filter materials are produced by ceramic mixing and shaping methods and subsequent sintering. To obtain a porosity of  greater than 30% and pore sizes of 1-20 xcexcm two basic methods are common use:
a) A coarse ceramic powder granulation of a mean grain size between 3-500 xcexcm is mixed with a fine ceramic powder granulation of a mean grain size between 0.01-7 xcexcm, whereby the fine-grained powder material envelopes the coarse-grained particles. The fine-grained material, due to its low melting point, acts as a flux during the sintering process. Such methods are described in, for example, EP 0 450 899, EP 0 585 152 or in the U.S. Pat. No. 5,223,318.
b) A fine ceramic powder granulation of a mean grain size between 0.01-7 xcexcm is mixed, shaped and sintered with organic material, whereby the organic material is burnt out during sintering and leaves open pores in the shaped body. Such methods are described in, for example, EP 0 354 721, and EP 0 549 873.
The use of the method to b) is problematic for environmental reasons and, moreover, this method does not ensure the required purity of the material for the porous filters. The prior art methods do not permit a manufacture of a ceramic filter material under use of TiO2-powder of sufficiently large grain sizes, whereby the filter material shall consist of nearly 100% TiO2 and shall have an open porosity of  greater than 20%. When TiO2-powder of a mean grain size of between 0.1-1 xcexcm is tempered, a sintering at a temperature of  greater than 1100xc2x0 C. starts which is accompanied by a structural densification and an increase of strength. Hence, it is not possible to achieve a sufficiently open porosity in this manner in ranges of temperature  greater than 1200xc2x0 C.
It is an object of the present invention to provide a method for producing monolithic, porous, ceramic shaped body which is adapted for use as support for inorganic and/or organic membranes for the separation of solids, liquids and gases by way of flow filtration, whereby
the material for these shaped bodies has to be TiO2 greater than 99.9% and
commercially available TiO2-powder can be utilized, and the shaped body shall have the following properties:
open porosity:  greater than 10%
mean pore size: 1-50 xcexcm, preferably 3-5 xcexcm,
mechanical strength:  greater than 20N/mm2, preferably  greater than 30N/mm2.
The object is realized by the features of the first claim. The strong structural compaction of the material in the course of sintering in the range of temperature  greater than 1200xc2x0 C. as typical for the prior art is eliminated in that a considerable part of the initial TiO2-powder is pre-sintered at temperatures  greater than 1200xc2x0 C., whereby the powder particles are sintered to agglomerates. In a subsequent process this pre-compacted material is reduced in size such that mean grain sizes of  less than 100 xcexcm will result. The thermally pre-compacted powder obtained in this manner is mixed with a further smaller part of the thermally not pretreated initial TiO2-powder, shaped and sintered at temperatures which lie below the pre-sintering temperatures. Thus, a sintering of the thermally not pretreated powder with the thermally pretreated powder is obtained, whereby the here occurring structural compaction is only the result of the sintering of the smaller part of the thermally not pretreated initial TiO2-powder. As a result of this proceeding, a monolithic porous ceramic shaped body is obtained exhibiting the properties which are to be obtained according to the task set by the present invention.